High-electric-field photocurrent in thin-film ZnS formed by pulsed-laser deposition

Photocurrent spectra of thin-film ZnS on glass fabricated by pulsed-laser deposition have been studied employing electric fields up to 20 kV/cm. The spectra show a shift towards lower energy at and beyond 10 kV/cm. By modeling the film absorption with the density of states and the Urbach rule, it is shown that, without the involvement of the Franz-Keldysh effect and excitonic transitions, the slope of the Urbach tail depends on the electric field, owing to impurity ionization. (C) 2003 American Institute of Physics.